The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
EP0060138, WO-A-00/34454 and WO-A-02/10070 describe the preparation of sulphonated polyHIPE polymers by soaking small pieces of material in concentrated sulphuric acid and raising the temperature to 60° C. over various time spans to achieve a desired degree of sulphonation. Sulphonation starts from the exterior causing the large particles to swell and making interconnecting holes smaller thereby reducing the penetration of the acid into the interior. This causes the formation of a hydrophilic skin with a hydrophobic core. Whilst it may be possible to obtain a fairly uniformly sulphonated polymer as a powder in which the diameter of the sulphonated particles is 1 cm or less, it is not possible to obtain uniformly sulphonated polymer in bulk. As the size of the pores is decreased, sulphonation becomes more difficult. This technique can not be used for large size monolithic polymers since the penetration of the acid into the polymer is restricted. After sulphonation, excess acid is washed away or neutralized and the polymer is dried. This process is less than straightforward and several washing cycles are typically required which generates excessive amounts of waste.
These drawbacks have been addressed in WO2004/005355 and WO 2004/004880 which describe the preparation of sulphonated polyHIPE polymers by incorporating dilute sulphuric acid as the dispersed (aqueous) phase during emulsification so that the acid is uniformly distributed within the pores when the emulsion is polymerized. After polymerization, the temperature is increased above 100° C. which results in sulphonation of the residual surfactants and a further rise in temperature above 140° C. results in sulphonation of the polymer. However, the sulphonation time is still long and the resulting polymers (although hydrophilic) are slow to take up water. Although the sulphonation time could be reduced (typically to 45 minutes) by increasing the temperature to 200° C., high temperatures are not desirable since the degree of sulphonation may be reduced.
The technique disclosed in EP0060138 uses highly concentrated sulphuric acid and is only suitable for small pieces of PHP. After sulphonation, excess acid has to be removed and PHP is washed with water and subsequently neutralized if desired.
Conventional nitration of PHP is time consuming and environmentally unfriendly due to the production of large amounts of dilute acid. Production of large monolithic structures is difficult due to poor penetration of acid into the polymer.